SHIELD is a US-led NATO counter-terrorism and intelligence agency, tasked with maintaining both national and global security with a focus on alien threats, magic, super-science as well as exotic weapons and materials. They oppose private corporations like AIM, cabals such as HYDRA, and even foreign operatives such as the Winter Guard.
Originally the Strategic Scientific Reserve, SHIELD was created from that organization in 1950 with the formation of NATO. SHIELD was organized as Strategic Homeland Investigation and Enforcement: Logistics Division. With many of the 'founding members' coming from the SSR, including Peggy Carter and Nick Fury Sr, there was a strong loyalty to the NATO states and a focus on enforcement and intelligence gathering, specifically against the remnants of HYDRA and the new Soviet threat: Leviathan.
SHIELD later added two specialized task forces. In 1983, Director Bridge authorized the creation of Task Force: WAND, specifically trained to deal with supernatural and paranormal threats to national security.
SHIELD has gone to ground to escape the public eye thanks to the machinations of HYDRA. Defections and betrayals at all levels have sorely tested the faith agents had in their comrades. After HYDRA and their allies were defeated, loyal SHIELD agents returned to the fold and now occupy the Triskelion once again.
SHIELD's founder Peggy Carter, would also be its first director with very nearly unanimous support from SHIELD's agents and allies alike. In 1985, Director Carter "retired" due to health issues. She was instead sealed in a cryogenic storage unit due to extreme cell degradation in response to a version of the Infinity Formula.
Following Carter's 'retirement' in 1985, George Washington Bridge would become the next director. Col. Frank Rhodes (father of Col. James Rhodes) was appointed to the Directorship in 1989. Director Rhodes retired in 2001 following an inquiry that revealed General Thaddeus Ross had accessed sealed files on Project: Rebirth.
After Rhodes stepped down, Colonel Nick Fury Jr. was nominated to the directorship. A consummate spymaster, he helmed SHIELD into the strange new world as metahuman populations surged and aliens made their presence on Earth officially known.
SHIELD claimed two specialized task forces over time. In 1983, Director Bridge authorized the creation of Task Force: WAND, specifically trained to deal with supernatural and paranormal threats to national security.
Over the years SHIELD has had clashes with both civilian and government entities. At times it has acted unlawfully, much to the chagrin of NATO, and has a storied history of espionage and clandestine military operations alike.
Currently, SHIELD is recognized as a global expert in handling and retrieving advanced or alien science and materials.